Reducing apps that create ongoing network traffic is a great way to improve battery life. Just remember that the screen is by FAR the largest user of the battery. I've also read that the battery life will improve after the first few cycles. (cycle == most to all drained, then charged.)
My favorite apps are ConnectBot (because there's a LOT I can do with ssh), NewsRob (best newsreader I've found; syncs with Google Reader), Google Listen for my podcasts...
Dolphin - excellent replacement browser with lots of cool features
Meebo IM - my favorite IM app
Advanced Task Manager - Best task manager app I've found. The usefulness of these things is
up for debate, though.
Seesmic - Twitter! It seems like a friendlier UI than twidroid
Google Voice - free sms and more
Transdroid - remotely control my torrents
handyCalc - the best, free calculator I could find (has graphing and other advanced stuff)
Subsonic - Of course we have Pandora and Last.fm, but Subsonic lets me
stream my own music from anywhere I have an internet connection! It works from any web browser or this sweet app. Really awesome app; highly recommended!
And Robo Defense is a really well done game. For killing time on your phone, especially if you dig tower defense... it's a good one.